FIG. 1 depicts a typical dimmer circuit 10 comprising a source of alternating-current (AC) power, or power supply, 12, a dimmer 14, and a load 16. The load 16 may be a lamp set comprising one or more lamps adapted to be connected between the hot and neutral terminals of a standard source of AC power. The lamp set may include one or more incandescent lamps and/or other loads such as electronic low voltage (ELV) or magnetic low voltage (MLV) loads, for example.
The power supply 12 supplies an AC waveform to the dimmer 14. The dimmer 14 regulates the delivery of electrical energy from the power supply 12 to the load 16. The dimmer may include a controllably conductive device 18 and a control circuit 20. The controllably conductive device 18 may include an input 22 adapted to be coupled to the power supply 12, an output 24 adapted to be coupled to the load 16, and a control input 26. The control circuit 20 may have an input 28 coupled to the input 22 of the controllably conductive device 18 and an output 30 coupled to the control input 26 of the controllably conductive device 18.
A typical, AC, phase-control dimmer regulates the amount of energy supplied to the load 16 by conducting for some portion of each half-cycle of the AC waveform, and not conducting for the remainder of the half-cycle. Because the dimmer 14 is in series with the load 16, the longer the dimmer 14 conducts, the more energy will be delivered to the load 16. Where the load 16 is a lamp set, the more energy delivered to the load, the greater the light intensity level of the lamp set. In a typical dimming scenario, a user may adjust a control to set the light intensity level of the lamp set to a desired light intensity level. The portion of each half-cycle for which the dimmer conducts is based on the selected light intensity level.
The controllably conductive device 18 may include a solid state switching device, which may include one or more triacs, which may be thyristors or similar control devices. Conventional light dimming circuits typically use triacs to control the conduction of line current through a load, allowing a predetermined conduction time, and control the average electrical power to the light. One technique for controlling the average electrical power is forward phase control. In forward phase control, a switching device, which may include a triac, for example, is turned on at some point within each AC line voltage half cycle and remains on until the next current zero crossing. Forward phase control is often used to control energy to a resistive or inductive load, which may include, for example, a magnetic lighting transformer.
Because a triac device can only be selectively turned on, a power-switching device, such as a field effect transistor (FET), a MOSFET (metal oxide semiconductor FET), or an insulated gate bipolar transistor (IGBT), for example, may be used for each half cycle of AC line input when turn-off phase is to be selectable. In reverse phase control, the switch is turned on at a voltage zero crossing of the AC line voltage and turned off at some point within each half cycle of the AC line current. A zero-crossing is defined as the time at which the voltage equals zero at the beginning of each half-cycle. Reverse phase control is often used to control energy to a capacitive load, which may include for example, an electronic transformer connected low voltage lamp.
The switching device may have a control or “gate” input 26 that is connected to a gate drive circuit, such as an FET drive circuit, for example. Control inputs on the gate input render the switching device conductive or non-conductive, which in turn controls the energy supplied to the load. FET drive circuitry typically provides control inputs to the switching device in response to command signals from a microcontroller. FET protection circuitry may also be provided. Such circuitry is well known and need not be described herein.
The microcontroller may be any processing device such as a programmable logic device (PLD), a microprocessor, or an application specific integrated circuit (ASIC), for example. Power to the microcontroller may be supplied by a power supply. A memory, such as an EEPROM, for example, may also be provided.
Inputs to the microcontroller may be received from a zero-crossing detector. The zero-crossing detector determines the zero-crossing points of the input AC waveform from the AC power supply 12. The microcontroller sets up gate control signals to operate the switching device to provide voltage from the AC power supply 12 to the load 16 at predetermined times relative to the zero-crossing points of the AC waveform. The zero-crossing detector may be a conventional zero-crossing detector, and need not be described here in further detail. In addition, the timing of transition firing pulses relative to the zero crossings of the AC waveform is also known, and need not be described further.
FIGS. 2A and 2B depict example prior-art dimmed waveforms. Shown in FIG. 2A is a single-frequency AC waveform having an amplitude profile v1(t) during a first half-cycle TH and an amplitude profile v2(t) during a second half-cycle TH. The absolute value of peak amplitude during each half cycle is A. Thus, the AC waveform delivered to the dimmer is the same for both half-cycles. Because the dimmer conducts for the same amount of time, tC, each half-cycle, tH, the amount of energy delivered to the load during the second half-cycle will be the same as the amount of energy delivered during the first half-cycle. Though this is usually acceptable, amplitude changes in the AC waveform may cause fluctuations in the amount of energy actually delivered to the load from half-cycle to half-cycle (e.g., the light level may fluctuate).
Shown in FIG. 2B, is a single-frequency AC waveform having an amplitude profile v1(t) during a first half-cycle TH and an amplitude profile v2(t) during a second half-cycle TH, wherein the absolute value of peak amplitude during the first half cycle is A1 and the absolute value of peak amplitude during the second half cycle is A2. As shown, the absolute value of peak amplitude, A2, of the waveform during the second half-cycle is different from (e.g., lower than) the absolute value of peak amplitude, A1, during the first half-cycle. Because the dimmer conducts for same amount of time, tC, each half-cycle, the amount of energy delivered to the load during second half-cycle is different from (e.g., less than) the amount of energy delivered to the load during the first half-cycle. Hence, given a constant dimmer-conduction time, tC, during each half-cycle, TH, the root-mean-square voltage delivered to the load will change as the amplitude of the AC waveform changes.
In certain installations, such as cruise ships or other such marine vessels, for example, other loads may be driven from the same power source that drives the lighting. For example, a ship's engines may be driven by the same power source that drives the ship's lighting. As energy is delivered to the engines, the voltage waveform delivered to the lighting is often corrupted. Sometimes, the waveform varies wildly between one half-cycle and the next.
In such an installation, if the lighting is dimmed using a technique that provides for delivering electrical energy to the lighting for a fixed amount of time each half-cycle, the variations in the waveform result in varying amounts of energy being delivered to the load every half-cycle. Consequently, the lighting flickers. Such a condition is undesirable.
It would be desirable, therefore, if apparatus and methods were available to deliver roughly the same amount of energy every half-cycle, regardless of variations in the amplitude of the AC waveform from half-cycle to half-cycle.